How to override all multiple keyframes at once?
I'm not sure it's what you want but let say you want your object to be rotated on its X axis all along the animation: Open the Graph Editor, lock all the tracks, unlock the track on which you want to make your operation (X Euler Rotation), select all (as everything except the X Euler Rotation is locked it will only select the X Euler Rotation curve) and move it on the Y axis of the Graph Editor (G Y) until it is rotated the way you want it to be.